1893.] Platinum Electrodes in Sulphuric A cid. 77 



the magnetising current. Amongst other points referred to in this 

 connexion is the time-lag in magnetisation, which is well shown by 

 the curve-tracer, and the effects are compared of the same cycle of 

 magnetic force gone through at various speeds. It is shown that in 

 solid bars 1'9 cm. in diameter, especially in soft iron, remarkable 

 evidences of time-lag are seen, even when the period of magnetic 

 reversal is as long as 3 sees. The work spent per cycle is a maximum 

 at a particular frequency, which in such bars is very low. 



The fourth and last section of the paper relates to the molecular 

 theory of magnetisation, and describes experiments made with groups 

 of small pivoted magnets. It is shown that the behaviour of such 

 groups, when exposed to the action of a variable magnetic field, 

 presents striking points of resemblance to the behaviour of iron or 

 steel under corresponding variations of magnetising force. Results 

 &re given which tend to confirm the theory. 



The particulars of the observations are set out in about forty sheets 

 of curves which accompany the paper. 



III. " Polarisation of Platinum Electrodes in Sulphuric Acid." 

 By JAMES B. HENDERSON, B.Sc. Communicated by LORD 

 KELVIN, P.R.S. Received June 10, 1893. 



This investigation was begun about the beginning of February, 

 1893, at the instigation of Lord Kelvin, and was conducted in the 

 Physical Laboratory of Glasgow University. The object of the in- 

 vestigation was to obtain the difference of potential between two 

 platinum electrodes immersed in a solution of sulphuric acid imme- 

 diately after the stoppage of a current which had been electrolysing 

 the solution, and to find how this difference varied with a variation in 

 the intensity of the current or in the strength of the solution. 



Former experiments by Buff (' Poggendorff,' vol. 130, p. 341, 1867) 

 smd Fromme ( 4 Wiedemann,' vol. 33, p. 80, 1888) have given for the 

 maximum polarisation with platinum wires of very small surface in 

 the electrolysis of dilute sulphuric acid 3*5 and 4*6 volts. 



Dr. Franz Richarz, in a paper " On the Polarisation of Small Elec- 

 trodes in Dilute Sulphuric Acid," read before the British Association 

 at Bath (1888), says of the above : 



" In these experiments the polarisation is calculated from measure- 

 ments of the intensity of the galvanic current during the electrolysis, 

 tacitly assuming that the resistance of the decomposition cell is in- 

 dependent of the intensity of the galvanic current. The correctness 

 of the supposition has not been proved. I tried experiments by 

 similar methods, and obtained yet greater values of the polarisation ; 

 it was calculated with a current density of 12 amperes per square 



